The favored research approach to achieve ultra high-quality compound semiconductor crystals is the bottom-seeded method—either the Vertical Gradient Freeze (VGF) or Bridgman technique. There are clear advantages of these methods over the more common top-seeded Czochralski (CZ) method, However, in some cases the crystal growth rate can be severely limited and in others the composition of the crystal can be non-uniform.
In the above bottom-seeded method, seed crystal 2 is positioned under growth crystal 4, in turn under melt 6, in vessel 8 as shown in FIG. 1.
In the VGF and Bridgman methods of crystal growth (FIG. 1), solidification is initiated either from a seed or from spontaneous nuclei at the bottom of a molten charge as opposed to the CZ method in which a seed is dipped into the melt from the top. Convective stirring due to thermally driven buoyancy, that is found in CZ melts, is not present in bottom-seeded melts, where the thermal gradient increasing from the bottom to the top of the melt provides thermal stability. This absence of strong convection, in fact, provides some of the advantages of the bottom-seeded methods over the CZ method (fewer dislocations and lower twinning probability). However, the absence of melt convection also generates the melt condition that limits the growth rate and causes compositional non-uniformity.
Generally the chemical composition of a solid is not precisely the same as that of the melt from which it freezes. This is known as non-congruent melting, as opposed to congruent melting, in which the composition of the solid and liquid phases are identical. For alloy crystals such as Ga1-xAlxSb, for example, the solid composition can be very different from that of the melt. Therefore during solidification one, or more of the constituent elements is rejected into the melt to form a boundary layer of liquid with a chemical make-up that is different from that of the bulk of the melt. This boundary layer builds up just adjacent to the crystal-melt interface. Strong convective stirring due to thermally driven buoyancy, is not present in bottom-seeded melts because the top is hotter than the bottom. Therefore the most effective transport mechanism in the boundary layer of a VGF melt is diffusion, which tends to be quite slow. This slow rate of diffusion determines the crystal growth rate limit. Typical growth rates for bottom-seeded melts are nearly an order of magnitude less than those for top-seeded CZ growth and therefore the cost of producing VGF crystals is greater. In addition, if there is virtually no mixing in the melt, the composition of the grown crystal can exhibit radial non-uniformity if the melt composition is not initially uniform from the center to the periphery.
Accordingly there is need and market for an improved growth process for the above alloy crystals that overcomes the above prior art shortcomings.
There has now been discovered a process for more controlled crystal growth to obtain more uniform crystals; both alloy crystals and non-congruently melting binary crystals.